Helix antenna

ABSTRACT

An antenna element is disclosed, having a ground plane ( 106 ), a helix ( 104 ) disposed above the ground plane ( 106 ), the helix ( 104 ) being connectable to a communications apparatus at a helix end ( 214 ) located near the ground plane ( 106 ), and a spiral ( 102 ) substantially centred on the axis ( 100 ) of the helix ( 104 ) the spiral ( 102 ) having an outer end thereof connected to the other helix end, said spiral ( 102 ) thereby terminating the antenna.

FIELD OF THE INVENTION

The present invention relates generally to antennas and, in particular,to helical antennas.

BACKGROUND

In Mobile Satellite System (MSS) networks, antenna performance at themobile terminal is critical in determining the performance of theoverall system. Considerable development work has thus been performedglobally relating to performance and implementation of antenna designsthat are suitable for terminals in such networks.

Patch antennas were initially considered because of their low physicalprofiles, and their theoretical peak gains of greater than 7 dB. Inpractical implementations, however, much lower peak gains were achieved.Furthermore, these antennas have narrow frequency bandwidth performance,and poor axial ratio performance at off-boresite angles, thus typicallylimiting their coverage to 25 degree elevation angles.

The aforementioned low antenna gain has been addressed by using phasedarray techniques which involve driving multiple antenna elements inparallel using a phased drive network. This enables higher overallantenna gain to be achieved while accepting lower gains from theindividual antenna elements. High gain phased-array antenna arrangementsusing patches, with either manual or automatic antenna pointing, cantypically provide between 9 dB and 18 dB of antenna gain. The phasedarray drive networks introduce undesirable losses into the antennaarrangements, however, and are complex to design across a broad range ofoperating frequency.

Low gain passive antennas using multifilar helices or patch elementshave been used in MSS networks, typically exhibiting antenna gains up to6 dB.

SUMMARY

An antenna concept disclosed herein provides a simple medium gainantenna, based on a low profile helix terminated with a spiral. Theantenna offers significantly higher antenna gain than patch antennaarrangements.

According to a first aspect of the invention, there is provided anantenna element comprising:

a ground plane;

a helix disposed above the ground plane, the helix being connectable toa communications apparatus at a helix end located near the ground plane;and

a spiral substantially centred on the axis of the helix the spiralhaving an outer end thereof connected to the other helix end, saidspiral thereby terminating the antenna.

According to another aspect of the invention, there is provided anantenna comprising:

a phased array feed network having an equipment feed-line for connectionto communication apparatus and a plurality of element feed-lines forconnection to a like plurality of antenna elements, said phased arrayfeed network being adapted to collectively connect said plurality ofantenna elements to the communication apparatus; and

said plurality of helix antenna elements arranged in a domino pattern,each said helix antenna element comprising a ground plane, and a helixdisposed above the ground plane, the helix being connectable to acommunications apparatus at a helix end located near the ground plane,each said helix antenna element being individually connectable at arespective helix end located near the ground plane to a respectiveelement feed-line of the phased array feed network to thereby connect tothe communications apparatus.

According to another aspect of the invention, there is provided anantenna comprising:

a ground plane;

a plurality of helix elements disposed above the ground plane, each saidhelix being connectable, via a respective feed line of an associatedphased array feed network to a communications apparatus, at a helix endlocated near the ground plane; and

a like plurality of spirals, each substantially centred on the axis ofthe corresponding one of the plurality of helix elements, said eachspiral having an outer end thereof connected to the other helix end ofthe corresponding one of the plurality of helix elements, said spiralthereby terminating the corresponding helix element.

According to another aspect of the invention, there is provided anantenna comprising:

a ground plane:

a plurality of helix elements disposed above the ground plane, each saidhelix being connectable, via a respective feed line of an associatedswitched element feed network to a communications apparatus, at a helixend located near the ground plane; and

a like plurality of spirals, each substantially centred on the axis ofthe corresponding one of the plurality of helix elements, said eachspiral having an outer end thereof connected to the other helix end ofthe corresponding one of the plurality of helix elements, said spiralthereby terminating the corresponding helix element.

According to another aspect of the invention, there is provided anantenna comprising:

a phased array feed network having an equipment feed-line for connectionto communication apparatus and a plurality of element feed-lines forconnection to a like plurality of antenna elements, said phased arrayfeed network being adapted to collectively connect said plurality ofantenna elements to the communication apparatus; and

said plurality of helix antenna elements being disposed above saidground plane and arranged in a rectangular grid pattern having a firstspacing between rows of said rectangular grid pattern and a secondspacing between columns of said rectangular grid pattern, each saidhelix antenna element being individually connectable at a respectivehelix end located near the ground plane to a respective elementfeed-line of the phased array feed network to thereby connect to thecommunications apparatus.

According to another aspect of the invention, there is provided a methodof impedance matching an antenna element wherein the antenna elementcomprises a ground plane, a helix disposed above the ground plane, thehelix being connectable to a communications apparatus at a helix endlocated near the ground plane, and a spiral substantially centred on theaxis of the helix the spiral having an outer end thereof connected tothe other helix end, said spiral thereby terminating the antenna, saidmethod comprising the steps of:

adjusting a distance, from the ground plane, of the helix end locatednear the ground plane to thereby adjust the impedance of a taperedtransmission line formed between the ground plane and a first quarterturn of the helix.

Other aspects of the invention are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be describedwith reference to the drawings, in which:

FIG. 1 shows the disclosed helix antenna;

FIG. 2 shows side and plan views of the antenna;

FIG. 3 shows a typical azimuth radiation pattern for the antenna;

FIG. 4A shows a switched antenna arrangement using the antenna;

FIG. 4B shows switch azimuth antenna gain patterns for the arrangementshown in FIG. 4A;

FIG. 5 shows an elevation pattern for the antenna;

FIG. 6 shows a feed network for a phased array antenna using helixantenna elements;

FIG. 7 shows inter-element distances for the array antenna of FIG. 6;

FIG. 8 shows an isometric view the antenna of FIG. 6;

FIG. 9 shows an antenna radiation pattern for the array antenna of FIG.8;

FIG. 10 depicts an array antenna using helix elements each having 20helical turns;

FIG. 11 shows an antenna radiation pattern for the array antenna of FIG.10;

FIG. 12 shows two antenna arrays disposed on a common ground plane;

FIG. 13 shows an isometric view of the transmit/receive array of FIG.12; and

FIG. 14 shows another array antenna using the helix antenna elements.

DETAILED DESCRIPTION INCLUDING BEST MODE

Where reference is made in any one or more of the accompanying drawingsto steps and/or features, which have the same reference numerals, thosesteps and/or features have for the purposes of this description the samefunction(s) or operation(s), unless the contrary intention appears.

FIG. 1 shows the disclosed helix antenna. The antenna comprises aconductive ground plane 106 above which is disposed a helical coil 104(alternately referred to in this description as a “helix”, a “helicalcoil” or the like) that is electrically terminated at the upper end ofthe helix 104 with a spiral 102. The helix antenna is depicted as havinga vertical axis 100.

In a preferred embodiment, the helical coil 104 comprises between 1.5and 3.5 turns. However, other numbers of turns can be used. Furthermore,the helix 104 is approximately one wavelength plus minus 10% of awavelength in circumference. In addition, the spiral 102 comprisesbetween 2 and 4 turns, in a flat configuration normal to the axis 100.

Although the ground plane 106 is depicted as having a circular shape inFIG. 1, in fact the extent of the ground plane 106 is not critical,provided that it has an area greater than two thirds of a wavelength indiameter.

FIG. 2 shows a side view 224 of the helix 104 and the spiral 102, andalso a plan view 232 thereof. Turning to the side view 224 the helix 104has a first end 214 that is disposed a distance 216 above the groundplane 106. This first end 214 of the helix 104 has a radial positionabout the axis 100 as depicted by a reference numeral 214′ in the planview 232.

The helix 104, when wound in a clock-wise direction produces right handcircular polarization, and when wound in a counter-clockwise direction,produces left hand circular polarization. The number of turns of thehelix can typically vary between 1.5 and 3.5, however the number ofturns can be varied outside these limits.

The helix 104 in FIG. 2 depicts one example of a helix being wound in acounter-clockwise direction commencing from the first end 214 andcomprises three and a quarter turns. The three and a quarter turnscomprise a first turn 212-210, a second turn 208-206, a third turn204-202, and a final quarter turn 200. The final quarter turn 200 of thehelix 104 runs from a radial position depicted by the arrow 214′ to aradial position depicted by the arrow 238 which is the upper end of thehelix 104. The upper end of the helix is connected to the outer end ofthe spiral 102 at a radial position 238.

The first quarter turn of the helix 104, which extends from the firstend 214 to a point 246, describes an angle 244 with respect to a dashedline 222. The remainder of the helix 104 is uniformly wound with a pitchangle 220, which can vary between 3 and 7 degrees, referred to thehorizontal reference line 222. The angle 244 can be adjusted to achievea desired impedance at the input of the helix 104. Although the angle isdepicted as being greater than the pitch angle 220, this is illustrativeonly, and other angles can be adopted according to the desiredimpedance. Furthermore, although an abrupt change between the angles 244and 220 occurs at the point 246 in FIG. 2, in practice a smooth angulartransition can be used.

The angle 244, together with the distance 216 of the helix first end 214from the ground plane 106 establishes a distance 228 which is located aquarter turn from the helix first end 214. The radial location of thedistance 228 is depicted by the reference numeral 238 in the plane view232. The one quarter turn segment of the helix 104 between 214 and 238forms a tapered transmission line with the ground plane 106. As noted,the distance 216 can be advantageously adjusted, for example byadjusting the angle 244, in order to match an input impedance of thehelix 104 as desired.

The helix 104 has a second end 242 that is situated, in the presentarrangement, three and a quarter turns from the first end 214 of thehelix 104. The spiral 102 is connected by an outer end there of to thesecond end 242 of the helix 104 at a radial location depicted by thereference numeral 238. The spiral 102 has a uniform inter-turn pitchdistance 236, and spirals inwards from the aforementioned outer end thatis connected to the second end 242 of the helix, to an inner end 234 ofthe spiral 102. Other types of spiral can also be used.

In a preferred arrangement the spiral 102 is located in a planehorizontal to the axis 100. The spiral 102 can however, in otherarrangements, be formed to have a conical shape pointing either upwardsor downwards.

Instead of a tapered transmission line being formed using the onequarter turn segment of the helix 104 between 214 and 238 and the groundplane 106, other impedance matching techniques such as quarter wavetransmission line matching sections can be used to connect the first end214 of the helix 104 to the intended communication apparatus therebyachieving the desired impedance matching.

The helix can be made of wire, wound on a low loss, low dielectricconstant former to support the helix and spiral. Alternately, the helixcan be etched in copper on a thin low loss dielectric film which is thenrolled to form a cylinder. Either method provides the necessarymechanical support for reliable operation and causes minimal disturbanceto the radiated wave.

This antenna element can be advantageously used in the frequency bandbetween 1 GHz and 8 GHz, however it can also be used outside thisfrequency band. Furthermore, the addition of the spiral 102 to terminatethe helix 104 is found to provide improved beam shaping and asignificant decrease in the antenna axial ratio. The antenna is ideallysuited for two-way communications via satellite to vehicles, vessels oraircraft. The antenna is a compact, low profile radiator exhibitingcircular polarisation, making it ideally suited for use where size andperformance are paramount such as in marine, aeronautical and landtransport services.

FIG. 3 shows a typical radiation pattern for the antenna of FIG. 1,which is seen to have high radiated power gain compared to other typesof antenna of similar dimensions.

The antenna of FIG. 1 has a low profile and a compact structure, therebymaking it an ideal radiator when used alone. It can also be used as aradiating element in an antenna array. A further advantage is that sincethe antenna provides higher individual antenna gains than, for example,patch antenna elements, the complex phasing networks that are requiredin order to drive multiple antenna elements in a phased array can bereplaced with a simple low loss antenna switching network in order toselect individual antenna elements according to the direction required.

FIG. 4A shows a partial switched-element arrangement 400. A generalomnidirectional antenna arrangement uses a series of 6 to 8 switchedelements comprising small antennas according to the arrangement of FIG.1, each antenna having a peak gain of at least 8 dBi after switchingnetwork losses. The depiction in FIG. 4A is directed to a single 90°quadrant between dashed lines 404 and 422 for ease of description. Threeantenna elements 406, 402 and 420 are disposed on an antenna housing418. The antenna elements 406, 402 and 420 are arranged so that theirbeam angles point in respective directions depicted by the dashed arrows404, 424 and 422. The antenna elements 406, 402 and 420 are connected byrespective feed lines 410, 416 and 414 to a switch arrangement 408, andthence by means of a connection 412 to the communications apparatus. Theapparatus can be a transmitter, a receiver, or a duplexer to which bothare connected for simultaneous transmit/receive.

It will be apparent that antennas according to the arrangement of FIG. 1can also be incorporated into a phased array by introducing a phasedarray feed network, instead of the switched feed network shown in FIG.5A, to thereby form a phased array antenna. This is described in moredetail in regard to FIGS. 6-14.

FIG. 4B depicts antenna beams 426, 430 and 434 that are associated withthe respective antenna elements 406, 402 and 420, the beams beingorientated along directions depicted by dashed arrows 404′, 424′ and422′ which correspond to respective directions 404, 424 and 422 in FIG.4A.

From an operational perspective the beam 426, for example, can beselected by switching the line 412 to the feed line 410 using theswitching arrangement 408. Similarly, the beam 434 can be selected byswitching the connection 412 to the feed line 414 using the switchingarrangement 408, and so on.

FIG. 5 shows an elevation pattern for the antenna shown in FIG. 1. Thepeak antenna gain is in excess of 9 dB, with broad coverage overelevation angles from 20 to 70 degrees.

The coverage at the zenith may be improved, if required, byincorporating an extra antenna element pointing to the zenith. Thiselement is connected to the switched array 400, for example, to providecoverage at the zenith.

A single helix with only approximate manual pointing of the antennawould also be attractive for non-mobile applications.

FIG. 6 shows a feed network 600 for a phased array antenna using fivehelix antenna elements as previously described, these antenna elementsbeing arranged in a domino configuration. The feed network depicted inFIG. 6 can be implemented in a number of different ways, includingmicrostrip and stripline, for example. When the array antenna in FIG. 6is used as a transmitting array, a signal 602 is input at 603 and flowsthrough a divider network 604. Energy flows to another divider 605 andis distributed along feed-lines 613 and 614 to respective helix antennaelements 601 and 608. The aforementioned helix antenna elements areshown in dashed form in order not to obscure details of the feed network600.

The input signal 602 is also distributed by the divider 604 to anotherdivider 606 which provides energy along a feed-line 616 to a helixantenna element 615. The divider 606 also provides signal power toanother divider 607 which provides signal along respective feed arms 610and 611 to respective helix antenna elements 609 and 612.

The feed network 600 is depicted in FIG. 6 as a component in atransmitting array, however it is apparent that the same antenna arraycan be used as a receive antenna array, in which case the arrow would bedirected in the opposite direction.

Equal feed-line lengths are used from the input 603 to each of theradiating elements 601, 608, 615, 609 and 612 in the arrangement 600.Furthermore, the energy delivered to each of the radiating elements isequal, and thus “uniform amplitude weighting” is used in the exampleshown. It is apparent, however, that variations in feed-line lengthsand/or amplitude weighting can be used to achieve specific array antennacharacteristics. The antenna elements 601, 608, 615, 609 and 612 aredisposed on a common ground plane such as 1211 in FIG. 13.

FIG. 7 shows a plan view 700 of the helix antenna elements 601, 608,615, 609 and 612 without the feed network 600 and in a domino pattern,i.e., pattern of domino pips, here five. The central helix antennaelement 615 is located at a radial inter-element distance 702 from theantenna element 601. The radial inter-element distance 702 can varybetween 0.51 and 2.51 at the frequency of operation of the antennaarray. Radial inter-element distances 705, 706 and 703 are equal to theradial inter-element distance 702. An inter-element distance 701 betweenthe helix antenna elements 601 and 608 can corresponding vary between0.71 and 3.51 at the frequency of operation of the antenna array.Inter-element distances 704, 708 and 707 are equal in length to theinter-element spacing 701. The inter-element spacings described inrelation to FIG. 7 are also applicable to the other array antennaarrangements described in relation to FIGS. 8, 10, 12, 13 and 14.

FIG. 8 show an isometric view 800 of five helix antenna elements801-805, each having five helical turns, that are disposed on a commonground plane with inter-element spacings as shown in FIG. 7. Each helixantenna element 801-805 is shown positioned on a ground plane segment806, however as noted, all the antenna elements 801-805 are mounted on acommon ground plane as will be shown in FIG. 13, for example.

FIG. 9 shows an antenna radiation pattern 900 for the array antenna ofFIG. 8. The gain of the array antenna is plotted against a verticalaccess 901 depicting power gain in dB and against a horizontal axis 902which represents angular deviation in degrees. The angular deviation ofthe horizontal axis 902 is measured with respect to a “boresite” axis ofthe array depicted in FIG. 8. For the array of FIG. 8, the boresite isthe axis of the helix 803, which is equivalent to the axis 100 inFIG. 1. Three antenna gain patterns, depicted by reference numerals903-905, are shown in FIG. 9, depicting the gain for the array antennaof FIG. 8 measured at relative lateral orientations of 0, 45 and 90degrees for the array antenna 800.

FIG. 10 depicts an array antenna 1000 similar to that shown in FIG. 8,but using helix elements each having 20 helical turns. It has been foundthat as the number of turns in the helix element increases, the antennaelement axial ratio decreases as well, thereby reducing the need for thespiral terminating element. The helix pitch angle 220 (see FIG. 2) whichfor low profile helix elements such as are illustrated in FIG. 2 canvary between 3 and 7 degrees referred to the horizontal reference line222, increases as the number of turns in the helix element increases,the pitch increasing to a value lying between 10-14 degrees. The array1000 comprises 5 helix antenna elements 1001-1005 which are disposed ina similar pattern to that shown in FIG. 8. The helix elements 1001-1005are disposed on a common ground plane depicted by 1006.

FIG. 11 depicts an array gain radiation pattern 1100 for the arrayantenna 1000 of FIG. 10. The radiation pattern is plotted against avertical axis 1101 depicting power gain in dB and a horizontal axis 1102depicting angular deviation in degrees from the boresite axis of thearray antenna 1000. Three gain patterns 1103-1105 are plotted in FIG.11, depicting the array antenna gain at relative lateral rotations of 0,45 and 90 degrees for the array antenna 1000.

FIG. 12 shows how two antenna arrays such as those depicted in FIGS. 8and 10 can be disposed on a common ground plane in order to act, forexample, as respective transmit and receive arrays. In FIG. 12 one arrayis depicted by large hashed circles 1201-1205, while the second array isdepicted by smaller hashed-circles 1206-1210. The array constituted bythe radiating elements 1206-1210 is laterally rotated with respect tothe array consisting of the radiating elements 1201-1205 in order tomaximise the inter-element spacing between elements of the two arrays.The inter-element spacing within each distinct array is consistent withthe inter-element spacings described in relation to FIG. 7. In FIG. 12the relative inter-element spacing for the two depicted arrays isdifferent since they operate at different frequencies, one frequencybeing allocated to the transmit function, and the other frequency beingallocated to the receive function.

FIG. 13 shows an isometric view 1300 of the transmit/receive array ofFIG. 12. The individual radiating elements 1201-1205 for the one arrayand 1206-1210 for the second array are shown mounted on a common groundplane 1211. The central radiating element 1208 is located within thecentral radiating element 1203.

FIG. 14 shows another arrangement 1400 of an array antenna using thehelix antenna elements described in relation to FIGS. 8, 10 and 13. InFIG. 14 helix radiating elements 1401-1416 are arranged in a rectangulargrid arrangement with horizontal inter-element spacings depicted by anarrow 1418 and vertical inter-element spacings depicted by an arrow1417.

INDUSTRIAL APPLICABILITY

It is apparent from the above that the arrangements described areapplicable to the mobile communication industry.

The foregoing describes only some embodiments of the present invention,and modifications and/or changes can be made thereto without departingfrom the scope and spirit of the invention, the embodiments beingillustrative and not restrictive.

1. An antenna element comprising: a ground plane; a cylindrical helixhaving a uniform pitch, the cylindrical helix being disposed above theground plane, the cylindrical helix being connectable to acommunications apparatus at a first helix end, the first helix end beinglocated near the ground plane; and a spiral spiraling inward in a flatconfiguration towards the axis of the cylindrical helix, the spiralhaving a first end thereof connected to a second helix end, the secondhelix end being the opposite end of the cylindrical helix to the firsthelix end, said spiral thereby terminating the antenna element whereinthe axis of the cylindrical helix is substantially perpendicular to theground plane, and the spiral lies in a flat plane that is substantiallyperpendicular to the axis of cylindrical helix.
 2. An antenna elementaccording to claim 1, further including a tapered transmission lineconnected between the communications apparatus and the first end of thecylindrical helix located near the ground plane.
 3. An antenna elementaccording to claim 1, wherein: the cylindrical helix has (a) between 1.5and 3.5 turns, (b) a pitch angle of between 3 and 7 degrees, and (c) acircumference of between 0.9 and 1.15 wavelengths; and the spiral hasbetween 1 and 4 turns.
 4. An antenna element according to claim 1,wherein: the cylindrical helix has (a) between 3.5 and 4.0 turns, (b) apitch angle of between 10 and 14 degrees, and (c) a circumference ofbetween 0.9 and 1.15 wavelengths; and the spiral has between 1 and 4turns.
 5. An antenna comprising: a switched element feed network havingan equipment feed-line for connection to communication apparatus and aplurality of element feed-lines for connection to a like plurality ofcylindrical helix antenna elements according to claim 1, said switchedelement feed network being adapted to connect a selected one of thecylindrical helix antenna elements to the communication apparatus; andsaid plurality of cylindrical helix antenna elements, said cylindricalhelix antenna elements being disposed above said ground plane, each saidcylindrical helix antenna element being individually connectable at arespective said first helix end located near the ground plane to arespective element feed-line of the switched element feed network tothereby connect to the communications apparatus.
 6. An antennacomprising: a phased array feed network having an equipment feed-linefor connection to communication apparatus and a plurality of elementfeed-lines for connection to a like plurality of cylindrical helixantenna elements according to claim 1, said phased array feed networkbeing adapted to collectively connect said plurality of cylindricalhelix antenna elements to the communication apparatus; and saidplurality of cylindrical helix antenna elements, said cylindrical helixantenna elements being disposed above said ground plane, each saidcylindrical helix antenna element being individually connectable at arespective said first helix end located near the graund plane to arespective element feed-line of the phased array feed network to therebyconnect to the communications apparatus.
 7. An antenna according toclaim 6, wherein: the plurality of cylindrical helix antenna elementsare arranged on a square grid in groups of five; and each of the groupsis arranged with (a) four members on grid intersection points of thegrid and (b) a fifth member at the centre of the four members.
 8. Anantenna comprising: a phased array feed network having an equipmentfeed-line for connection to communication apparatus and a plurality ofelement feed-lines for connection to a like plurality of cylindricalhelix antenna elements, said phased array feed network being adapted tocollectively connect said plurality of cylindrical helix antennaelements to the communication apparatus; and said plurality ofcylindrical helix antenna elements arranged in a pattern of domino pips,on a square grid in groups of five wherein each said group is arrangedwith (a) four members of the group on gird intersection points and (b)the fifth member of the group at the centre of said four members, eachsaid cylindrical helix antenna element comprising a ground plane and acylindrical helix having a uniform pitch disposed above the groundplane, each said cylindrical helix antenna element being individuallyconnectable at a respective first cylindrical helix end located near theground plane to a respective element feed-line of the phased array feednetwork to thereby connect said cylindrical helix antenna element to thecommunications apparatus, wherein each said cylindrical helix antennaelement further comprises a spiral spiraling inwards in a flatconfiguration towards on the axis of the cylindrical helix thel spiralhaving a first end thereof connected to a second helix end being theopposite end of the cylindrical helix to the first helix end, saidspiral thereby terminating the antenna wherein the axis of thecylindrical helix is substantially perpendicular to the ground plane,and the spiral lies in a flat plane that is substantially perpendicularto the axis of the helix.
 9. An antenna according to claim 7, wherein:in regard to a said group of five elements the radial inter-elementspacing between the centre antenna element and antenna elements on saidcorners grid intersection points is between 0.5λ and 2.5λ at thefrequency of operation of the antenna.
 10. An antenna having twoantennas according to claim 7, wherein: a centre cylindrical helixantenna element of a first of said two antennas is co-located with acentre cylindrical helix antenna element of a second of said twoantennas; and the first of said two antennas is laterally rotated withrespect to the second of said two antennas, said lateral rotation beingabout a common axis of the co-located centre cylindrical helix antennaelements to thereby change inter-element spacing between antennaelements of said two antennas.
 11. An antenna comprising: a groundplane; a plurality of cylindrical helices disposed above the groundplane, each said cylindrical helix being connectable, via a respectivefeed line of an associated phased array feed network to a communicationsapparatus, at a respective first helix end located near the groundplane; and a like plurality of spirals, each spiraling inwards in a flatconfiguration towards the axis of the corresponding one of the pluralityof cylindrical helices, said each lateral spiral having a first endthereof connected to a second helix end of the corresponding one of theplurality of helices, said second helix end being the opposite end ofthe cylindrical helix to the first helix end, said spiral therebyterminating the corresponding helix; wherein the axis of the cylindricalhelix is substantially perpendicular to the ground plane, and the spirallies in a flat plane that is substantially perpendicular to the axis ofthe helix.
 12. An antenna comprising: a ground plane; a plurality ofcylindrical helices disposed above the ground plane, each saidcylindrical helix being connectable, via a respective feed line of anassociated switched element feed network to a communications apparatus,at a respective first helix end located near the ground plane; and alike plurality of spirals, each spiraling inward in a flat configurationtowards the axis of the corresponding one of the plurality ofcylindrical helices, said each spiral having a first end thereofconnected to a second helix end of the corresponding one of theplurality of cylindrical helices, said lateral spiral therebyterminating the corresponding helix; wherein the axis of the cylindricalhelix is substantially perpendicular to the ground plane, and the spirallies in a flat plane that is substantially perpendicular to the axis ofthe helix.
 13. An antenna comprising: a phased array feed network havingan equipment feed-line for connection to communication apparatus and aplurality of element feed-lines for connection to a like plurality ofcylindrical helix antenna elements, said phased array feed network beingadapted to collectively connect said plurality of cylindrical helixantenna elements to the communication apparatus; and said plurality ofcylindrical helix antenna elements according to claim 1, said helixantenna elements being disposed above said ground plane and arranged ina rectangular grid pattern having a first spacing between rows of saidrectangular grid pattern and a second spacing between columns of saidrectangular grid pattern, each said cylindrical helix antenna elementbeing individually connectable at a respective first helix end locatednear the ground plane to a respective element feed-line of the phasedarray feed network to thereby connect to the communications apparatus.14. A method of impedance matching a cylindrical helix antenna elementwherein the cylindrical helix antenna element comprises a graund plane,a cylindrical helix having a uniform pitch disposed above the groundplane, the cylindrical helix being connectable to a communicationsapparatus at a first helix end located near the ground plane, and aspiral spiraling inward in a flat configuration towards the axis of thecylindrical helix the spira1 having a first end thereof connected to asecond helix end, said second helix end being the opposite end of thecylindrical helix to the first helix end, said spiral therebyterminating the cylindrical helix antenna, wherein the axis of thecylindrical helix is substantially perpendicular to the ground plane,and the spiral lies in a flat plane that is substantially perpendicularto the axis of the helix, said method comprising the steps of: adjustinga distance, from the ground plane, of the first helix end located nearthe ground plane to thereby adjust the impedance of a taperedtransmission line formed between the ground plane and a first quarterturn of the cylindrical helix.
 15. An antenna according to claim 8,wherein: having regard to a said group of five elements, the radialinter-element spacing between the centre antenna element and antennaelements on said grid intersection points is between 0.5λ and 2.5λ atthe frequency of operation of the antenna.
 16. An antenna having twoantennas according to claim 8, wherein: a centre cylindrical helixantenna element of a first of said two antennas is co-located with acentre cylindrical helix antenna element of a second of said twoantennas; and the first of said two antennas is laterally rotated withrespect to the second of said two antennas, said lateral rotation beingabout a common axis of the co-located centre cylindrical helix antennaelements to thereby change inter-element spacing between antennaelements of said two antennas.